1. Field of the Invention
The present invention relates to power supplies in general and more particularly to voltage regulating circuits suitable for processing variable a.c. line voltages to generate rectified d.c. output voltages.
2. Prior Art
Most power supplies, especially the ones that are used in computers and other precision electronic devices are required to provide a fixed voltage at a given current. Usually, the allowed deviation at full load, is within the range of .+-.5%. Preferably, the power supply should be low-cost and consume a relatively small amount of power.
A typical power supply includes a power transformer with a primary winding which is connected to an a.c. input line voltage and one or more secondary windings disposed relative to the primary winding. Rectifying circuits are connected to the secondary windings. The rectifying circuits process the a.c. voltages appearing on the secondary winding to provide a desired d.c. output voltage across a bulk capacitor.
One of the problems which a designer faces is that the input a.c. line voltage varies over a wide range. An obvious solution is to design a regulating circuit which dissipates a relatively large amount of power. Thus, as the input AC line voltage varies from minimum to maximum, more power is consumed within the regulating circuit to provide a desired voltage at a given current. Such a design is unacceptable because the regulating unit usually dissipates a relatively large amount of energy and its cost is also relatively high. It is believed that the high cost stems from the fact that the components which are used in such devices are specified to match the high energy that has to be dissipated rather than the power which the devices output. It is common knowledge that for most designs unit cost and power rating are closely related. Thus, as the power rating increases, the unit cost increases and vice versa. Therefore, it is desirable to design a device with components dissipating close to the output power rating rather than with components rated for higher power due to circuit inefficiency.
The prior art has recognized the problem associated with the above-described design and sets forth alternative designs. U.S. Pat. No. 3,921,059 is an example of the prior art alternative design. In that patent multiple triac taps connected to secondary windings of a power transformer are switched to provide a range of output voltages. A control circuit including a shift register and optical isolator drivers are used to switch the triacs.
U.S. Pat. No. 4,454,466 describes a power supply in which switched primary windings provide a variable voltage which is processed by a series regulator to output a fixed voltage to a load. An up-down counter circuit arrangement is used for driving switches that select the primary windings which are needed to provide a desired output voltage.
IBM Technical Disclosure Bulletin (Vol. 13, No. 6, Nov. 1970, pp. 1516-1517) and U.S. Pat. No. 4,090,234 describe a voltage regulating circuitry in which diodes and SCR taps are made selectively conductive to effectively vary the turn ratio of the secondary winding of a power transformer.
Even though the above devices work well for their intended purpose, they are plagued by problems which adversely affect their use. Probably the most pressing problem is that the automatic tap settings have to be latched. This means that the tap switching to select coils cannot be done instantaneously, since the counters and/or latches which provide the latching function cannot be changed instantaneously. In other words, the rate at which the latching elements are changed is the rate at which the coils can be switched. Counters and other latching elements can only be changed on every clock cycle. Thus, the coils can only be switched on a clock cycle basis only. Any attempt to switch coils during a clock cycle is prohibited. However, there are several sensitive devices (such as computers, etc.) that require regulating circuits in which the coils must be switched instantaneously. As to those devices, the prior art regulating circuits and/or power supplies cannot be used.
It is noted that the regulating circuits of the Technical Disclosure Bulletin and the '234 patent uses SCRs instead of latches. However, one of the inherent characteristics of an SCR is that it must remain in a desired state (i.e., conductive or non-conductive) for a complete clock cycle before it can be changed. Thus, in that regard, the SCR operates as a latch. In addition, the '234 patent requires a separate power supply for driving the SCR. This requirement makes the circuit more complicated and increases its cost. The increased cost also affects the '466 patent since switching is done on the primary side of the transformer and as a result the components are overdesigned in order to handle the high current and/or voltage in the primary windings. Also, some of the prior art references use opto-isolation and/or triacs in the switching circuits. These components are expensive and increase the overall cost of the power supply.